While everybody outside of the US has been able to mix and match iPhone and wireless operator, the iPhone was still tied to a single carrier in the US. Today, Verizon and Apple announced the much-hyped Verizon iPhone 4. While the rest of the world collectively yawns, this is good news for American consumers, since there will be more competition, and thus, more choice. Edit: Eh, the other way around. First choice, then competition.

It doesn't really matter who's number one, but it does matter that a lot of other people use the same phone as you (or at least the same OS) because without the numbers, there will be fewer apps and fewer accessories. One of the main reasons to use the iPhone is the vast support for the music playing aspects in cars, audio equipment, etc. If it were a niche device, there wouldn't be that benefit.